The removal of H.sub.2 S and other acid gases such as CO.sub.2 from sour industrial gases with various amines and the regeneration of the amines with the production of a concentrated off gas containing H.sub.2 S and CO.sub.2 which is useful as a feed gas to a Claus sulfur recovery unit is well known in the art as is illustrated by U.S. Pat. No. 4,085,199 dated Apr. 18, 1978.
The present art thus provides for selective removal of H.sub.2 S with various solvents, however the degree of selectivity is less than 100%. If an acid gas or off-gas of increased H.sub.2 S content over the off-gas composition of a first regenerator is desired, one must install a complete second plant, a third, fourth, et cetera until the desired level of H.sub.2 S in the acid gas is achieved. Each stage requires a separate contactor and regenerator.
The extension of the prior art, one stage selective H.sub.2 S removal, would thus be a multiple stage contact and regeneration. This method will require low energy (restricted to solvent circulation and solvent regeneration) but will be a high capital project due to the multiple tower configuration and the inflexibility of said configuration which will have a very narrow operating range in regard to feed composition and feed volume.
The lower range for the H.sub.2 S/CO.sub.2 feed to a Claus unit is 20 to 25% by volume of H.sub.2 S. However, there are special techniques that claim feasible and economically viable operation with as little as 5% H.sub.2 S in the Claus feed but with a relatively great increase in both capital and treating costs. There is thus need in the art for a process which will increase the concentration of H.sub.2 S in the Claus feed gas with relatively lower energy requirements and capital investments since in so doing the Claus unit efficiency and overall treating costs will be improved.
In South African Pat. Nos. 7800645 and 7800644 to Shell Research, there are disclosed methods for increasing or adjusting the level of H.sub.2 S in a H.sub.2 S--CO.sub.2 natural gas or refinery stream so that the stream can be used as a feed stream to a Claus unit.
A limitation of the foregoing Shell processes is that (1) to achieve the desired H.sub.2 S concentration of the Claus feed gas they require more energy i.e., heat input, than the present invention, (2) variations in the feed gas compositions are not perceived by the control units, and (3) in contrast to the previous art, the present invention can process low pressure gas streams as well as high pressure streams.
The present invention provides for automatic H.sub.2 S analysis of the rich amine absorbent which controls the H.sub.2 S content of the feed gas to a Claus unit independently of the variations in acid gas concentration of the feed gas.